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Random Access - Lseek in C

Introduction

lseek is a system call that is used to alter the location of the read/write pointer of a file descriptor. The lseek system call basically moves the current read/write location to anywhere within the referenced file.

The Input and output are normally sequential in the concept of file descriptors. Each read and write operation in this concept takes place at a position in the file right after the previous one. The file can be read or written in any random order in case it is necessary or required.

The lseek plays a vital role here by providing a way to navigate around in a file without reading or writing any information or data.

System call

long lseek (int fd, long offset, int origin);

The above piece of code is a system call which sets the current position in the file whose descriptor is fd to offset, which is taken relative to the location specified by origin. Here, offset is the number of bytes to move the selector.

The concluding read or write will begin at that position. The origin can be 0(zero),1 (one), or 2(two). It is numbered so, in order to specify that offset is measured from the beginning (top), from the current position (where current pointer points), or from the end of the file respectively.

Example

In order to be adjoined to a file, we need to seek to the end before writing the following. This can be achieved by using the following piece of code;

lseek (fd, 0L, 2);  // seek to the end before writing

Note: In order to get back to the beginning, the term (“rewind”) is used,

lseek (fd, 0L, 0);

Note: In the above piece of code,the ‘0L’ argument here can be written as (long) 0 or just as (zero) 0 if the declaration of lseek is properly done.

Use of lseek in C

The lseek is quite useful in C programming and implementation of file concepts like fopen or getc. With the lseek(random access), it is possible to treat files like arrays (more or like large arrays). This may help the programmer but it costs in terms of slower access.

For example:

The following function reads the nth byte of a file and copies it to another file using the lseek.

Program 1:

#include <stdio.h>    //header files 
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>


void func(char arr[], int n)
{


int f_write = open ("start.txt", O_RDONLY);


int f_read = open("end.txt", O_WRONLY); 


int count = 0;


while (read(f_write, arr, 1))
{
		
if (count < n)
{
			
lseek (f_write, n, SEEK_CUR);
write (f_read, arr, 1);
count = n;
}
else
{
count = (2*n);
lseek(f_write, count, SEEK_CUR);
write(f_read, arr, 1);
}
}
close(f_write);
close(f_read);
}


int main()
{
	char array[50];  // Driver code
	int n;    //declaring a variable ‘n’ of type ‘int’
	n = 5;  //value assigned to n


	
	func(array, n);  //function call
	return 0;
}

Example 2:

#include <sys/types.h>    //header files


int get(int fd, long pos, char *buf, int n)
{


    if(lseek(fd, pos, 0)>=0)
    return read(fd, buf, n); 
    else
    return -1;  //return ‘-1’ in case of error
}

Code explanation:

In the above code, the function reads any number of bytes from any arbitrary (or random) place in a file.

It returns the number read. In case an error occurs, it returns ‘-1’.

Return value of lseek

The return value of lseek is a long that gives the new position in the file. In case an error occurs, it returns ‘-1’. The standard library function fseek is similar to lseek. Of Course, there are some similarities as well;

  • The first argument is a file *
  •  In case an error occurs, it returns ‘-1’.

fseek function in C

fseek() function is another similar library function to lseek in C programming which is used to move file pointer position to the given location.

SEEK_CUR – Seek_Current is used to move file pointer positions to a given location.

SEEK_END – Seek_End is used to move file pointer position to the end of file.

Drawback of lseek

Is Lseek thread safe? The original whence, exactly as passed to the lseek(2) system call.

Note: This function is not thread safe, because it shares a return buffer across all threads, and many other functions in this library

Note 2: TheRETURN VALUE

Upon successful termination;

The lseek() or random access returns the resulting offset location as measured in bytes from the top or beginning of the file. In case an error occurs, it returns ‘-1’.